The invention concerns a method for regulating the concentrations of components of additives in a printing process liquid, wherein the actual concentrations of the components are determined and the measured components are redosed to obtain predetermined desired concentrations, as well as a method for regulating the concentrations of components of additives in a printing process liquid using a measuring means for measuring the concentrations of at least part of the components in the process liquid, comprising a control loop and a means for redosing at least part of the components.
In offset printing machines, the printing plate is wet with an aqueous liquid using a so-called dampening system, such that image areas accept the ink in a subsequent processing step, whereas the image-free areas repel the ink. In addition to water, the aqueous liquid often contains an alcohol mixture, in most cases isopropanol, as well as a chemical mixture of up to approximately twenty substances (referred to below as an additive). The additive is dosed in concentrations of between 1 and 8 vol. % and the alcohol is added in concentrations of between 0.5 and 20 vol. %. The concentration of water is therefore between 72 and 98.5 vol. %. If optimized additives are used, alcohol is sometimes completely omitted. In this case, the additive is also called an alcohol substitute. The optimized additive either completely or partially assumes the function of the isopropanol. Additives substantially contain the following substance groups:                surface-active substances such as higher alcohols and tensides which reduce, in particular, the dynamic surface tension, and are used, possibly partially, as an isopropanol substitute,        buffers, in particular phosphate and citrate buffers which maintain the pH value of the dampening solution at a constant value in a range between 4.8 and 5.3,        wetting agents such as e.g. glycerine which make the printing plates hydrophilic,        antioxidants as corrosion protection, glycols and glycol ether which act as solubilizers and keep the above-mentioned substance groups in the aqueous solution, and        substances having a germ-killing effect.        
One major problem in offset printing is the insufficient up-time of the very expensive printing machine, which is typically only approximately 80% and is therefore characterized by long down-times. One could e.g. save approximately 35,000.00 Euros per year if the pure productive time per day of a so-called 64 page rotary offset printing line could merely be increased by an average of two minutes. New, intensive practical examinations have clearly shown that the insufficient up-time of offset printing machines is essentially due to the undefined, unknown physical and chemical composition of the process liquid which, to date, cannot be measured and therefore cannot be regulated. These experiments showed, in a particular and in a paradoxical manner, that even if a predetermined volumetric mixture of the water and additive components is exactly realized, e.g. through precise control of two dosing pumps injecting e.g. volumes of 97 vol % water and 3 vol % additive into the process liquid, a much lower value is actually present in the process liquid circuit, e.g. 0.8 vol % of additive. Even more surprising, analyses have shown that the original percentage composition of the individual components of the additive in the dampening solution circuit do not correspond to the originally targeted composition which was injected by the dosing pumps through controlled feeding. Processes take place (“cannibalistic effects”), with which the components of the additive are consumed during the printing process to a greater or lesser degree despite the fact that they are added periodically in accordance with targeted concentration proportions. Current, conventional offset printing technology of feeding the additive concentrate in the form of one single chemical mixture which contains all required chemical components with precisely predetermined concentrations and whose composition depends on the application of pressure, i.e. roller offset, sheet-fed offset or newspaper printing and on the type of machine, paper, ink, in the dampening solution circuit of a printing machine, is an inadequate procedure which does not meet modern requirements for high up-time in the offset process. Although these disadvantages can be compensated for to a certain degree in printing with alcohol through the addition of higher concentrations of isopropanol, as is current practice, this method cannot be regarded as a technical solution for the future, since isopropanol, being a solvent and volatile component (VOC=volatile organic compound), is prohibited in offset printing in many US states, subject to strict laws for emission reduction in Europe, and even fined in Switzerland with a penalty tax, the so-called “Lenkungsabgabe”, which is detrimental to the economics of the printing process. For political environmental reasons and, in particular, to protect the health of the printers at their workplace, isopropanol or other solvents must be substantially reduced or completely eliminated in future printing processes. The concentrations of alcohol in the dampening solution are currently generally between 6% and 20% and facilitate the use of so-called film dampening devices in roller and sheet-fed offset printing. In accordance with prior art, the film dampening devices comprise several rollers which are coated with rubber mixtures and/or metals and which are rotated together in contact with each other under slight pressure to transport the dampening solution, in the form of a film of adjustable film thickness, to the printing plate. This transport process is facilitated by the addition of isopropanol due to the reduction of surface tension of the liquid film caused thereby. In addition to conventional film dampening devices, contact-free operating systems, in particular, spray dampening devices operating with nozzles, or dampening devices comprising rollers jacketed with plush are also used. In these cases, the dampening means is transported without continuous liquid film, and use of alcohol may therefore be omitted. The new inventive method is also of great importance for conventional designs, since it permits optimum composition of chemicals in the dampening solution.
To meet the legal constraints regarding the ban of isopropanol, other solvents have been marketed, in particular in the U.S.A. This has not been the case in Europe, since this solution does not eliminate the use of solvents. Moreover, some of the other solvents are assumed to cause cancer or be detrimental to health and therefore do not constitute an alternative to alcohol.
A real alternative to alcohol are the so-called tensides which achieve comparable advantages with regard to the wetting properties of the dampening solution on the rollers of the dampening device. It must be noted, in particular, that tensides are not VOCs. Experience has shown that these positive tenside properties may be utilized only if the required targeted concentrations can be accurately met. In the currently used conventional alcohol-free methods, tensides produce undesired foams and emulsification of ink and dampening solution which reduces quality, such that, in many practical applications, printing without alcohol fails and must be replaced by printing with alcohol. This is further complicated by the fact that tensides in a chemical multi-component mixture often only dissolve with great difficulty, which requires the addition of solvents into the additive concentrate to prevent separation, i.e. deposit on the bottom of the additive container of the supplier. This difficulty is also easily solved by the inventive method, which provides the possibility of applying only those chemical substance components which are absolutely necessary for the printing process. Since the sheet speeds of modern printing machines are constantly increasing, increasingly precise measurement and dosing of the individual chemical components are required. The inventive method is therefore essential to printing without alcohol. This is supported by the fact that, with exactly the same printing machine, the composition of the individual components of the additive must be variable—depending on the printing orders i.e. on the paper, the particular inks required by the specific customer, the specially used rubber blanket, the roller coating, etc. This is only possible with the new method described herein. This is particularly true since there is no single conventional additive anywhere in the world which permits printing without alcohol under all conditions that occur in a printing machine. This explains why printing managers want to repeatedly test other additive formulations to realize their printing orders. Nevertheless, each chemical formulation is a compromise and is therefore optimum only for a limited range of printing orders. In total, the current conventional procedure is very expensive and renders printing without alcohol impossible in practice, despite the above-mentioned legal regulations in Europe.
Conventionally, dosing means are used for generating the process liquid by volumetrically mixing the two or three components through control under fixed predetermined conditions and introducing them into the liquid circuit of the printing machine in accordance with the respective consumption, i.e. in accordance with discharge of the liquid to the paper being printed. In addition to mixing stations, which are operated by hand, systems with conventional dosing pumps are also currently used. A severe disadvantage of these systems is that neither malfunctions of the mixing means nor changes in the physical and/or chemical composition, e.g. due to chemical reactions or absorption or desorption processes by the printing ink, paper, the pipe conduit or machine modules, can be defined. In particular, evaporation processes produce considerable concentration errors in these classical dosing methods. The sensors for detecting the electrical conductance which are currently used as sole control instruments are unsuitable for quantitative measurement of the concentration of the respective additive or substitute, due to the strong and varying soiling of the process liquid. Moreover, the important conducting chemical components of the additives which permit printing cannot be detected through conductance measurements, since these substances cannot be dissociated in water. The pH probe which has been introduced more or less as a standard in offset printing can at most be used as an indicator shortly before the functional collapse of the printing process, since the required strong chemical buffering of the process liquid e.g. using citric acid, prevents change of the pH value even for large variations in the chemical composition.
The object of the present invention is therefore the readjustment to the respective target values through continuous measurement and regulation of the composition of the dampening solution, i.e. through continuous redosing of the individual, differently decreasing chemical components or selected groups of components, to increase the up-time of the offset printing process to values of competing gravure printing, i.e. to approximately 90 to 95%.